HR: 0800h
AN: T41C-0707    [Abstracts]
TI: Apatite volatile barometry at Volcan Irazu, Costa Rica
AU: * Boyce, J W
EM: jwboyce@alum.mit.edu
AF: School of Earth and Space Exploration, Arizona State University, Box 871404, Tempe, AZ 85287, United States
AU: Hervig, R L
EM: hervig@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, Box 871404, Tempe, AZ 85287, United States
AB: The 1723 and 1963 basaltic-andesite eruptions of Irazu volcano, Costa Rica, contain abundant fluorapatite phenocrysts with both OH-apatite and Cl-apatite components. The OH and Cl content of apatite may provide information about volatile contents and processes occurring in a magma chamber prior to eruption. SIMS measurements of Cl and H in apatite phenocrysts from these two eruptions demonstrate systematic differences in volatile composition in apatite from 1723 to 1963: Chlorine and hydroxyl contents are higher in apatite erupted in 1723 (Cl = 7000 - 8500 ppm; OH = 4000 - 5500 ppm) than in those from the 1963 eruption (Cl = 4500 - 6000 ppm; OH = 2500 - 4200 ppm). In a qualitative sense, these results are in agreement with the data of Benjamin et al. (JVGR, in press), which demonstrate a decline in Cl and H from 1723 to 1963 in melt inclusions from the same samples. However, in order to interpret apatite volatile content in terms of quantitative magma volatile composition, knowledge of apatite/melt partition coefficients is required. Mathez and Webster (GCA, v. 65, no.5, 2005) have demonstrated that apatite partition coefficients are likely a function of melt chemistry for Cl and F, and their data suggest that the same is true for OH partitioning between silicate melts and apatite. Using the apatite/melt partition coefficients based on the data of Mathez and Webster, without accounting for compositional differences between the basaltic andesites of Irazu and the basaltic melts represented in their experiments, results in model magmatic volatile contents for Irazu that are far in excess of measured melt inclusion values for both Cl and H2O for both 1963 and 1723 eruptions (Benjamin et al.). This lack of agreement is not surprising, as the two melt compositions (the experiments and the natural samples) are quite different. We have developed a multivariate linear regression model for Cl and OH partition coefficients allowing estimation of partition coefficients for Irazu melts. This model suggests that Cl in Irazu magmas decreased from 4500 ppm to 2400 ppm from the 1723 to 1963 eruption events, while H2O decreased from 3.4 wt% to 1.6 wt%. These estimates agree very closely with the melt inclusion data of Benjamin et al. Although this model for apatite is strongly dependent on the scarce partitioning data that exists, it demonstrates that microanalyses of volatile elements in apatite can provide robust quantitative information about magmatic Cl and H2O. However, it also emphasizes the need for additional experiments on a wide range of melt compositions in order to study diverse magmas.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1065 Major and trace element geochemistry
DE: 3618 Magma chamber processes (1036)
DE: 3694 Instruments and techniques
SC: Tectonophysics [T]
MN: 2007 Fall Meeting